The object of the invention is an energy-saving internal sizing air-jet mill having a pregrinding chamber for fine grinding of preferably various carbides, silicates, oxides, ores, pigments or elastic materials, as well as for surface treatment and/or cryogenic grinding of the same.
The air-jet mills known according to the present state of art, could be divided into five basic types. The first type is characteristic of the situation where grinding comes about by accelerating the material to high speed through a nozzle and impacting it against a so-called anvil. This apparatus provides adequate grinding; however, due to high specific energy consumption its operation is not economical and the lining experiences much wear thereby causing high contamination of the ground product.
In order to eliminate such contaminating effect, it is often the practice to use linings from the same basic material as the material which is to be ground. The best known version of this type now in use is the air-jet mill of the Vortex system provided with an outer sizer or separator, ceramic lining and anvil. The other type of the air-jet mill widely used is the so-called Majec mill. Here, comminution takes place by the autogenous grinding effect of grains impacting against each other by the acceleration generated through two nozzles facing one another. This operation, however, exhibits energy losses and thus very poor comminution efficiency. The nozzles can carry comparatively small amounts of grains and vortices occur also due to the effect of the opposite air jets thus fairly reducing the number of collision of grains. Known examples of this type are: West German Patents DE No. 2543691 C2 and DE No. 2523471 C2.
The third air-jet mill, the so called Micronizer type is the one which has been used most predominantly. The essence of its operation is that grinding takes place in the discusshaped grinding chamber under the effect of pressurized gas issuing from peripheral jet pipes. The gas jets first contact a circle in the outer third part or half of the grinding area. Material to be ground enters the grinding space in a vertical plane crossing the tangent of this circle, however, at an angle of 60.degree. to the vertical passing through the top of the grinding space.
According to the theory of the designers, the grains greater in size than a predetermined dimension are circulating along this tangential circle, the smaller grains, that is, the ground end product discharge from the facility past obstructing dam entrained in the exhaust and, the coarser grains, under the effect of the pressurized gas from the peripheral nozzles, collide with each other and circulates until their dimensions are reduced to or below the required level. Under actual working conditions the operation of the facility does not meet the conditions of the above theoretical operation, yet the type is widely used as the unit presenting the best efficiency. Several patented inventions exist on the above apparatus, e.g. U.S. Pat. No. 3,726,484, SF-33960, DE No. 3201778 C1. These technical solutions represent the combination of the double-jet mill, the anvil type and the micronizer, where the coarse product is returned to the grinding space, or by applying a anvil-type pregrinder an attempt is made, with little success, to improve the fineness of grinding. Therefore, up to now, the unchanged basic type provided with some kind of liner is most frequently used in the industry.
With the fourth type of the air-jet mills, the goal was the increase of mill output by a method which did not cause the shortening of the path of free movement of particles.
In favour of this, the volume of grinding space and the number of nozzles has been increased, increasing thereby the output of mill relative to its unit volume, however, the efficiency of energy utilization has been decreased and the extent of wear also increased. This type of mill is known as the Jet-O-Mixer or Reductionizer. Addressed to the reduction of wear, the design of the Double-Impact-Mill appeared on the market. In the return branch of the upper part of the mill a so-called directional-change-sizer and further grinding nozzles have been applied in some cases. The achievement of a grain size of 1 .mu.m by the use of these types of mills was not realized.
Fluid bed air-jet-mills may be included in the fifth type (e.g. DE No. 3140294 C2) where frequency of collision of particles and thus the efficiency of grinding is increased by the use of four nozzles of larger diameter, when compared to the previously mentioned methods, and which are operated opposite to each other and are located in the bottom part of a large container. The nozzles operate to fluidize the entire amount of material in the container with air flow entraining the finer grains to be passed through a rotating sizer in the top section of the container. Meanwhile, the coarser fraction slips back down the wall of the container for repeated grinding.
The above device possesses reasonably good grinding and sizing efficiency, however, it is not suitable for fine (below 10 .mu.) grinding, partly because, due to the short path length of the particles (high density), the resultant impact energy of the particles is small, partly because even the speed of rotation of the revolving part of the sizer, controlling the fineness of the end product, cannot be increased beyond a certain limit. Other disadvantages of this design lie in the condition that the revolving part of the sizer is exposed to high wear and, due to the high pressure in the grinding space, charging of the material can only be carried out by the use of an involved sluice system. Based on the knowledge obtained from the aforementioned types of mills, it can be established that the efficiency of air-jet facilities is favourable only if the particles possess high energy and the probability of impacting is also high, however, as the free path length required for particles to become accelerated shortens, the impact energy also diminishes, therefore, there is a compromise forming the basic problem in the operation of air-jet facilities: either to increase the free path length and make the ground product finer along with diminishing efficiency of the mill, or to increase the number of impacts which while resulting in a coarser product, improves the grinding efficiency and performance of the mill.